Fatty acids are one of the principal components of lipids. In vivo, fatty acids are bonded to glycerin via an ester bond to form lipids (fats and oils) such as triacylglycerol. Further, many animals and plants also store and utilize fatty acids as an energy source. These fatty acids and lipids stored in animals and plants are widely utilized for food or industrial use.
For example, higher alcohol derivatives that are obtained by reducing higher fatty acids having approximately 12 to 18 carbon atoms are used as surfactants. Alkyl sulfuric acid ester salts, alkyl benzene sulfonic acid salts and the like are utilized as anionic surfactants. Further, polyoxyalkylene alkyl ethers, alkyl polyglycosides and the like are utilized as nonionic surfactants. These surfactants are used for detergents, disinfectants, or the like. Cationic surfactants such as alkylamine salts and mono- or dialkyl-quaternary amine salts, as other higher alcohol derivatives, are commonly used for fiber treatment agents, hair conditioning agents, disinfectants, or the like. Further, benzalkonium type quaternary ammonium salts are commonly used for disinfectants, antiseptics, or the like. Furthermore, vegetable fats and oils are also used as raw materials of biodiesel fuels.
Further, most of long-chain fatty acids having 18 or more carbon atoms, particularly long-chain polyunsaturated fatty acids into which a plurality of unsaturated bonds are introduced (hereinafter, also referred to as “PUFA”) are known to be essential fatty acids which are unable to be synthesized in vivo in animals. Accordingly, such PUFA is particularly useful in nutritional use and utilized as physiologically functional food and the like.
A fatty acid synthetic pathway of plants is localized in the chloroplast. In the chloroplast, an elongation reaction of the carbon chain is repeated starting from an acetyl-ACP (acyl-carrier-protein), and finally an acyl-ACP (a composite consisting of an acyl group being a fatty acid residue and an ACP) having 16 or 18 carbon atoms is synthesized. A β-ketoacyl-ACP synthase (hereinafter, also referred to as “KAS”) is an enzyme involved in control of chain length of the acyl group, among enzymes involved in the fatty acid synthetic pathway. In plants, four kinds of KASs having different function respectively, namely KAS I, KAS II, KAS III and KAS IV are known to exist. Among these, KAS III functions in a stage of starting a chain length elongation reaction to elongate the acetyl-ACP (or acetyl-CoA) having 2 carbon atoms to the β-ketoacyl-ACP having 4 carbon atoms. In the subsequent elongation reaction, KAS I, KAS II and KAS IV are involved. KAS I is mainly involved in the elongation reaction to the palmitoyl-ACP having 16 carbon atoms, and KAS II is mainly involved in the elongation reaction to the stearoyl-ACP having 18 carbon atoms. On the other hand, it is believed that KAS IV is involved in the elongation reaction to medium-chain acyl-ACP having 6 to 14 carbon atoms.
Furthermore, the long-chain fatty acids having 18 or more carbon atoms, particularly PUFA is reputedly synthesized by a number of desaturase or elongase outside the chloroplasts.
As mentioned above, fatty acids are widely used in various applications. Therefore, attempts have been made on improving productivity of the fatty acids or the lipids in vivo by using hosts such as plants. Furthermore, applications and usefulness of the fatty acids depend on the number of carbon atoms (chain length) or unsaturated bonds (degree of unsaturation) thereof. Therefore attempts have been made also on controlling the number of carbon atoms or unsaturated bonds of the fatty acids.
In general, enhancement of desaturase or elongase is considered to be effective in improving the productivity of PUFA, and these enzyme groups have been identified from various organisms (see Patent Literature 1). For example, desaturase or elongase derived from algae, being one kind of microalgae, belonged to the genus Nannochloropsis, on which attention is focused as a next-generation production source for fats and oils is known to be usable for synthesis of the long-chain fatty acids (see Patent Literature 2 and 3).
Thus, a method of effectively producing the lipids rich in desired long-chain fatty acids (particularly PUFA) in oleaginous organisms is in demand in the technical field.